Cast iron material

ABSTRACT

Provided is a cast iron material from which excellent friction characteristics can be obtained. Provided is the cast iron material containing C and Fe as a composition, and further containing Cr as the composition in 1.0 to 3.5% in terms of mass %. The cast iron material is used in a sliding component sliding under an environment of lubricating oil to which an additive containing Mo as a constituent element, such as MoDTC, is added. Cr contained in the cast iron material promotes a decomposition reaction of the additive containing Mo added to the lubricating oil to form a film of molybdenum disulfide, the film having low friction. Thus, the fiction can be reduced.

TECHNICAL FIELD

The invention relates to a cast iron material having excellent frictioncharacteristics.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is based upon and claims the benefit of priorityto Japanese Application No. 2017-221363, filed Nov. 16, 2017, the entirecontents of which are incorporated herein by reference.

BACKGROUND ART

Cast iron has been widely used as a sliding material for a slidingcomponent of an internal combustion engine, or the like because ofhaving favorable wear resistance and seizure resistance.

For example, Patent Literature 1 discloses, as cast iron for a cylinderliner in which wear resistance is improved, a material which has acomposition containing C: 3.0 to 3.5%, Si: 1.5 to 2.5%, Mn: 0.5 to 1.0%,P: 0.2 to 0.5%, S: 0.12% or less, Cr: 0.1 to 0.5%, B: 0.09 to 0.18%, Cu:0.4 to 1.0%, and Mo: 0.1 to 0.5% in terms of mass %, and a remaindercomposed of Fe and inevitable impurities, and is formed into a structureconfigured by including a base phase mainly composed of pearlite,dispersing a hard phase composed of steadite and a boron compound in 14to 22% in terms of an area proportion, and simultaneously dispersingflake graphite so as to have an average graphite distance of 9 to 15 μm.According to the cast iron for the cylinder liner, a corrosion loss canbe reduced to a half in comparison with conventional cast iron, and aseizure limit load can be improved to about one and a half times largerthan the conventional cast iron.

In addition, for example, Patent Literature 2 discloses highwear-resistant Cr cast iron that has a chemical composition containingC: 2.7 to 3.3%, Si: 0.2 to 1.0%, Mn: 0.4 to 2.0%, Cr: 18 to 25%, Mo: 0.5to 4%, Ni: 0.5 to 3%, and N: less than 0.2% in terms of mass %, and aremainder composed of Fe and impurities; and has a structure composed of30 to 40 area % of crystallized carbide and a matrix surrounding thecarbide, in which the matrix is mainly formed of martensite, and aquenched structure partially containing retained austenite is tempered,and fine precipitated carbide having a particle size of 1 rpm or less inan equivalent circular diameter is dispersed in the matrix, and a totalamount of the fine precipitated carbide is adjusted to 3.0 to 14 area %based on the total structure. According to the high wear-resistant Crcast iron, a wear loss can be reduced to a half in comparison with theconventional cast iron.

In addition, for example, Patent Literature 3 discloses flake graphitecast iron containing A-type graphite including an existence form inwhich graphite is distributed disorderly and uniformly with nodirection, and having a chemical composition containing C: 2.8 to 4.0%,Si: 1.2 to 3.0%, Mn: 1.1 to 3.0%, P: 0.01 to 0.6%, and S: 0.01 to 0.30%in terms of mass %, and a remainder composed of Fe and inevitableimpurities, in which a ratio of a Mn content to a S content (Mn/S) is ina range of 3 to 300. According to the flake graphite cast iron, tensilestrength can be improved to about 1.2 to 2 times larger than theconventional cast iron, and can also obtain favorable machinability.

Further, for example, Patent Literature 4 discloses flake graphite castiron which has a composition containing C: 2.4 to 3.6%, Si: 0.8% or moreand less than 2.8%, Mn: 1.1 to 3.0%, and further P: 0.01 to 0.6% and B:0.001 to 0.2%, or further S: more than 0.01% and 0.15% or less, and onekind or two or more kinds selected from Cu, Cr, Mo, and Ni in 0.1 to6.0% in total, one kind or two or more kinds selected from W, V, and Nbin 0.01 to 5.0% in total, and one kind or two or more kinds selectedfrom Sn: 0.3% or less and Sb: 0.3% or less, in terms of mass %, andfurther has a structure in which carbide is dispersed in 8% or less interms of area %. According to the flake graphite cast iron, tensilestrength can be improved to about 1.5 times larger than the conventionalcast iron.

CITATION LIST Patent Literature

-   Patent Literature 1: JP 2006-206986 A-   Patent Literature 2: JP 2009-007597A-   Patent Literature 3: JP 2013-117071 A-   Patent Literature 4: JP 2014-062318 A

Non-Patent Literature

-   Ushioda et al, Effect of Low Viscosity Passenger Car Motor Oils on    Fuel Economy Engine Tests, SAE international, 2013-01-2606

SUMMARY OF INVENTION Technical Problem

However, reduction of viscosity of engine oil has progressed in recentyears in order to reduce stirring resistance of the engine oil, therebycausing an increase of direct contact of metal with each other accordingto high load operating conditions and the like, and a rise of friction(see, Non-Patent Literature 1, for example), which has a possibility ofcausing seizure or the like. Therefore, a friction modifier is added tothe engine oil to reduce the friction. As the friction modifier that hasbeen most frequently used so far, MoDTC (molybdenum dithiocarbamate ormolybdenum dialkyldithiocarbamate) is available. However, an actionmechanism of the friction modifier containing Mo (molybdenum) as anadditive, or a reason why an effect of the friction modifier isdifferent depending on a material has not been specified yet. Therefore,it is desired to elucidate the mechanisms, thereby developing the effectof the friction modifier at maximum to reduce the fiction. Further, alarge amount of materials in which hardness is increased in order toimprove the characteristics such as wear resistance has been provided sofar to have a problem of having difficulty in working and poorproductivity.

The invention has been made based on such problems, and contemplated forproviding a cast iron material from which excellent fictioncharacteristics can be obtained.

Solution to Problem

A cast iron material according to the invention contains C and Fe as acomposition, and Cr as the composition in 1.0 to 3.5% in terms of mass%, and includes graphite as a structure, and is used in a slidingcomponent sliding under an environment of lubricating oil containing Mo.

Advantageous Effects of Invention

According to a cast iron material of the invention, the cast ironmaterial contains Cr in 1.0% or more in terms of mass %, and thereforeactive Cr exposed onto a surface due to sliding promotes a decompositionreaction of an additive contained in lubricating oil, and a film ofmolybdenum disulfide, the film having low friction, can be formed.Accordingly, friction can be reduced, and wear can also be reduced, andseizure or the like can also be suppressed. In addition, Cr is adjustedto 3.5% or less in terms of mass %, and therefore the cast iron materialis suppressed from becoming unnecessarily hard, and can be easilyworked.

Moreover, the cast iron material is adjusted to contain at least Si fromthe group consisting of Si, Cu and Ni as the composition to adjust acontent thereof to 2 to 6.5% of Si, 0 to 1.5% of Cu, and 0 to 1.5% of Niin terms of mass %, and therefore the cast iron material can be adjustedto suitable hardness, and can be easily worked.

Further, if the cast iron material is arranged to be used in a slidingcomponent of the engine, the friction can be reduced and fuel efficiencycan also be improved.

BRIEF DESCRIPTION OF DRAWINGS

This application file contains at least one drawing executed in color.Copies of the patent application publication with color drawing(s) willbe provided by the Office upon request and payment of the necessary fee.

FIG. 1 shows one example of a structure of a cast iron materialaccording to one embodiment of the invention, and is an opticalmicrograph of the structure, showing the structure including flakegraphite.

FIG. 2 shows another example of a structure of a cast iron materialaccording to one embodiment of the invention, and is an opticalmicrograph of the structure, showing the structure including spheroidalgraphite.

FIG. 3 is a transmission electron micrograph showing one example of across-sectional structure of friction marks after conducting a frictiontest on a cast iron material according to the invention.

FIG. 4 is an enlarged diagram of a frame portion in FIG. 3, and atransmission electron micrograph showing formed molybdenum disulfide.

FIG. 5 is a characteristic diagram showing, in comparison, a frictioncoefficient under an environment of lubricating oil to which MoDTC isadded with regard to the cast iron material in each of Examples andComparative Examples.

FIG. 6 is another characteristic diagram showing, in comparison, afriction coefficient under an environment of lubricating oil to whichMoDTC is added with regard to the cast iron material in each of Examplesand Comparative Examples.

FIG. 7 is a list showing a difference in friction characteristicsdepending on a composition of a cast iron material under an environmentof lubricating oil to which MoDTC is added with regard to the cast ironmaterial in each of Examples and Comparative Examples.

FIG. 8 is a list showing results obtained after preparing a test piecehaving a circular arc cross section by using the cast iron material ineach of Examples and Comparative Examples, and conducting a frictiontest under an environment of lubricating oil to which MoDTC is added.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the invention will be described in detailwith reference to drawings.

FIG. 1 and FIG. 2 each show one example of a structure of a cast ironmaterial according to one embodiment of the invention. FIG. 1 shows thestructure including flake graphite as graphite, and FIG. 2 shows thestructure including spheroidal graphite as the graphite. The cast ironmaterial is used in a sliding component sliding under an environment oflubricating oil containing molybdenum (Mo), and in particular, can bepreferably used in the sliding component of an engine. In addition, aterm “the lubricating oil containing Mo” means the lubricating oil towhich an additive containing Mo as a constituent element is added, andmeans the lubricating oil to which organic molybdenum such as MoDTC isadded as the additive, for example. The additive containing Mo as theconstituent element in this manner is used as a friction modifier, forexample.

The cast iron material according to the present embodiment containscarbon (C) and iron (Fe), and further chromium (Cr) as the composition.C reinforces a pearlite base in a base material, and simultaneouslycrystallizes the graphite to improve slidability, wear resistance andseizure resistance. A content of C is preferably from 2.0 to 6.5% interms of mass %, for example. The reason is that, if the content of C issmall, the flake graphite is not crystallized to affect workability orthe like.

Cr is a material for promoting a decomposition reaction of the additivecontaining Mo as the constituent element, such as MoDTC, added to thelubricating oil to form a larger amount of a film of molybdenumdisulfide, the film having low friction. If the film of molybdenumdisulfide is formed, friction can be reduced, and wear can also bereduced, and seizure or the like can also be suppressed.

A mechanism according to which Cr decomposes the additive containing Moas the constituent element, such as MoDTC, is estimated as describedbelow. First, if an oxide layer existing in a surface layer of the castiron material is scraped by the friction caused by sliding, active metal(such as Fe and Cr) is exposed. In addition, the additive such as MoDTCcontained in the lubricating oil is decomposed by heat, and molybdenumoxysulfide (MoS_(2-x)O_(x)) being an intermediate product exists in thelubricating oil. Ionization tendency of the metal is specified by aseries: Cr>Fe≈Mo, and therefore Cr which is further easily oxidized thanFe deprives molybdenum oxysulfide existing in the lubricating oil ofoxygen into chromium oxide. Meanwhile, molybdenum oxysulfide which isdeprived of oxygen is formed into molybdenum disulfide (MoS₂) to formthe film on the cast iron material. In addition, when the friction testis conducted on the cast iron material according to the presentembodiment under the environment of lubricating oil to which MoDTC isadded, it has been confirmed that a reactional film as shown in FIG. 3is formed on the surface of the cast iron material after testing, andlayer films of molybdenum disulfide are formed as shown in FIG. 4.

A content of Cr is preferably in the range from 1.0 to 3.5% in terms ofmass %. The reason is that, if the content thereof is less than 1.0% bymass, the effect of forming the film of molybdenum disulfide by Crcannot be sufficiently obtained, and if the content thereof becomes morethan 3.5% by mass, the cast iron material becomes unnecessarily hard,thereby being difficult in being easily worked.

Further, the cast iron material according to the present embodimentpreferably contains at least silicon (Si) from the group consisting ofsilicon (Si), copper (Cu) and nickel (Ni) as the composition. Si is amaterial having an effect on suppressing Cr from forming carbide tosuppressing the cast ion material from being hard, thereby facilitatingworking. A content of Si is preferably adjusted to 2 to 6.5% in terms ofmass %. The reason is that, if the content thereof is less than 2% bymass, the cast iron material becomes excessively hard in several cases,and if the content thereof becomes more than 6.5% by mass, the cast ironmaterial becomes brittle and has a possibility of losing strength.

In a manner similar to Si, Cu and Ni also have an effect on suppressingCr from forming carbide; but the cast iron material may contain or neednot contain Cu and Ni. A content of Cu is preferably adjusted to 0 to1.5%, and a content of Ni is preferably adjusted to 0 to 1.5% in termsof mass %. The reason is that, within the above ranges, an effect ofsuppressing Cr from forming carbide, and avoiding being excessivelyhard, thereby facilitating working can be obtained.

In addition, the hardness of the cast iron material is preferablyadjusted within the range from HB200 to HB380 in terms of Brinellhardness. The reason is that, if the harness thereof becomes more thanHP380, machinability is deteriorated, and working becomes difficult.

As the structure of the cast iron material according to the presentembodiment, the base material is composed of the pearlite base, andflake graphite or spheroidal graphite, and crystallized carbide aredispersed in the base material. Further, Cr is dispersed in the basematerial, thereby promoting a reaction of the friction modifiercontaining Mo to form molybdenum disulfide. Thus, excellent frictioncharacteristics can be obtained.

The cast iron material can be obtained by melt-forming a melt having theabove-described composition by an ordinary melt-forming method using acupola, an electric furnace or the like, casting the resultant materialby using a publicly-known casting method and solidifying the resultantmaterial, for example.

Thus, according to the present embodiment, the cast iron materialcontains Cr in 1.0% or more in terms of mass %, and therefore active Crexposed on the surface by sliding can promote the decomposition reactionof the friction modifier contained in the lubricating oil to form thefilm of molybdenum disulfide. Accordingly, the friction can be reduced,the wear can also be reduced, and the seizure or the like can also besuppressed. In addition, Cr is adjusted to 3.5% or less in terms of mass%, and therefore the cast iron material can be suppressed from becomingunnecessarily hard, and can be easily worked.

Further, the cast iron material is adjusted to contain at least Si fromthe group consisting of Si, Cu and Ni as the composition, and thecontent is adjusted to 2 to 6.5% of Si, 0 to 1.5% of Cu; and 0 to 1.5%of Ni in terms of mass %. Therefore, the cast iron material can beadjusted to suitable hardness and can be easily worked.

Further, if the cast iron material is arranged to be used in a slidingportion of an engine part or a driving part, the friction can be reducedand fuel efficiency can be improved.

EXAMPLES Examples 1-1 to 1-6 and Comparative Examples 1-1 to 1-6

A cast iron material was prepared, in which the cast ion material had acomposition containing 2.6% of C, 2.6% of Cr, 4.4% of Si, and 1.0% of Cuin terms of mass %, and a remainder composed of Fe and impurities, andhad a structure in which a base material was formed of a pearlite base,and flake graphite and crystallized carbide were dispersed in the basematerial. On the thus obtained cast iron material, a reciprocatingsliding friction test was conducted under an environment of lubricatingoil. As the lubricating oil, test oil to which MoDTC was added was usedfor all, and kinds thereof were changed in Examples 1-1 to 1-6. As amating material, bearing steel (SUJ2) was used. The test was conductedfor 30 minutes by adjusting a load to 80 N, a frequency to 10 Hz, and atemperature to 80° C., and a friction coefficient during stabilizationwas measured.

As Comparative Examples 1-1 to 1-6, a friction test similar to the testin the present Example was conducted by using a cast iron materialhaving a composition without containing Cr and containing 3.0% of C, and2.2% of Si in terms of mass %, and a remainder composed of Fe andimpurities, and having a structure in which a base material was formedof a pearlite base, and flake graphite and crystallized carbide weredispersed in the base material, and a function coefficient was measured.As lubricating oil, the same lubricating oil was used between Example1-1 and Comparative Example 1-1, between Example 1-2 and ComparativeExample 1-2, between Example 1-3 and Comparative Example 1-3, betweenExample 1-4 and Comparative Example 1-4, between Example 1-5 andComparative Example 1-5, and between Example 1-6 and Comparative Example1-6, respectively.

The t results obtained are shown in FIG. 5. As shown in FIG. 5,according to the present Examples, the friction coefficient was able tobe reduced for all. More specifically, it was found that the frictioncan be reduced in the cast iron material containing Cr as thecomposition.

Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-4

A cast iron material was prepared, in which the cast ion material had acomposition containing 3.2% of C, 2.5% of Cr, and 4.9% of Si in terms ofmass %, and a remainder composed of Fe and impurities, and had astructure in which a base material was formed of a pearlite base, andspheroidal graphite and crystallized carbide were dispersed in the basematerial. On the thus obtained cast iron material, a reciprocatingsliding friction test was conducted under an environment of lubricatingoil. As the lubricating oil, test oil to which MoDTC was added was usedfor all, and kinds thereof were changed in Examples 2-1 to 2-4. As amating material, bearing steel (SUJ2) was used. The test was conductedfor 30 minutes by adjusting a load to 80 N, a frequency to 10 Hz, and atemperature to 80° C., and a friction coefficient during stabilizationwas measured.

As Comparative Examples 2-1 to 2-4, a friction test similar to the testin the present Example was conducted by using a cast iron materialhaving a composition without containing Cr and containing 3.5% of C, and2.4% of Si in terms of mass %, and a remainder composed of Fe andimpurities, and having a structure in which a base material was formedof a pearlite base, and spheroidal graphite and crystallized carbidewere dispersed in the base material, and a friction coefficient wasmeasured. As lubricating oil, the same lubricating oil was used betweenExample 2-1 and Comparative Example 2-1, between Example 2-2 andComparative Example 2-2, between Example 2-3 and Comparative Example2-3, and between Example 2-4 and Comparative Example 2-4, respectively.

The results obtained are shown in FIG. 6. As shown in FIG. 6, accordingto the present Examples, the friction coefficient was able to be reducedfor all. More specifically, it was found that the friction can bereduced in the cast iron material containing Cr as the composition.

Examples 3-1 to 3-4 and Comparative Examples 3-1 to 3-3

A cast iron material having a composition in which an amount of Cr, anamount of Si, an amount of Ni, and an amount of Cu were different fromeach other, and a remainder was composed of Fe and impurities wasprepared. On the thus obtained cast iron material, a friction test wasconducted under an environment of lubricating oil containing MoDTC. Asthe friction test, a ball-on-disk type friction test was conducted, andas a mating material, bearing steel (SUJ2) was used. The test wasconducted for 30 minutes by adjusting a load to 80 N, a frictional speedto 0.5 m/s, and a temperature to 80° C., and a friction coefficientduring stabilization was measured.

As Comparative Examples, in Comparative Example 3-1, a materialcontaining Cr in an amount less than 1.0% was used, and in ComparativeExample 3-2 and 3-3, materials each containing Cr in an amount more than3.5% were used. In Examples 3-1 and 3-2, materials each containing Cr inabout 1.0% were used, and in Examples 3-3 and 3-4, materials eachcontaining Cr in 2.3 to 2.55% were used. An amount of Si and an amountof Cu were different from each other between Examples 3-1 and 3-2 andbetween Examples 3-3 and 3-4.

The results obtained are shown in FIG. 7. As shown in FIG. 7, it wasfound that, while the friction coefficient is high in ComparativeExample 3-1 in which the amount of Cr is low, the friction coefficientwas low to be a half or less in Examples 3-1 to 3-4. It was found that,while the friction coefficient is low in Comparative Examples 3-2 and3-3 in which the amount of Cr is large, the cast iron material becomesexcessively hard.

Examples 4-1 to 4-3 and Comparative Examples 4-1 and 4-2

A test piece having a circular arc cross section was prepared by using acast iron material having a composition in which an amount of Cr, anamount of Si, an amount of Ni, and an amount of Cu were different fromeach other, and a remainder was composed of Fe and impurities. On thethus obtained test piece, a function test was conducted under anenvironment of lubricating oil containing MoDTC. The test was conductedfor 30 minutes by adjusting a load to 80 N, a frequency to 10 Hz, and atemperature to 80° C., and a friction coefficient during stabilizationwas measured.

As Comparative Examples 4-1 and 4-2, a material containing Cr in anamount less than 1.0% was used, and as Examples 4-1 to 4-3, a materialcontaining Cr in 1.0 to 3.5% was used. In Examples 4-1 to 4-3, theamount of Si and the amount of Cu were different from each other.

The results obtained are shown in FIG. 8. As shown in FIG. 8, it wasfound that, in comparison with Comparative Examples 4-1 and 4-2 in whichthe amount of Cr is less than 1.0%, the friction coefficient is reducedby about 20% in Examples 4-1 to 4-3 in which the amount of Cr is from1.0 to 3.5%. From the results, it was revealed that the friction isreduced by use of the cast iron material according to the invention alsoin a cylinder liner sliding along a piston ring as a mating material inassuming an automobile engine.

As described above, the invention is described by exemplifying theembodiments, but the invention is not limited to the embodimentsdescribed above, and can be modified in various manners. For example,the composition of the cast iron is specifically described in theembodiments described above, but the composition may contain any otherelement. For example, any other element includes Mg, Mn, S, P or thelike.

The invention is also understood as a sliding mechanism including a pairof sliding members each having a sliding surface sliding with eachother, and lubricating oil interposed between the sliding surfacesfacing each other, in which at least one of the sliding surfaces isformed of a cast iron material containing carbon (C) and iron (Fe) as acomposition, and further chromium (Cr) in 1.0 to 3.5% in terms of mass%, and including graphite as a structure, and the lubricating oilcontains molybdenum (Mo) as an additive.

1. A cast iron material, comprising: carbon (C) and iron (Fe) as acomposition; and chromium (Cr) as the composition in 1.0 to 3.5% interms of mass %; and including graphite as a structure, wherein the castiron material is used in a sliding component sliding under anenvironment of lubricating oil containing molybdenum (Mo) as anadditive.
 2. The cast iron material according to claim 1, comprising atleast silicon (Si) from the group consisting of silicon (Si), copper(Cu) and nickel (Ni) as the composition, wherein a content thereof isfrom 2 to 6.5% of Si, from 0 to 1.5% of Cu, and from 0 to 1.5% of Ni interms of mass %.
 3. The cast iron material according to claim 1, whereinBrinell hardness is from HB200 to HB380.
 4. The cast iron materialaccording to claim 1, wherein a film containing molybdenum disulfide isformed on a surface by sliding.
 5. The cast iron material according toclaim 1, wherein the cast iron material is used in a sliding portion ofan engine part and a driving part.